Composite polyester/PVA barrier films

ABSTRACT

Polyester-based, biaxially oriented composite films having good transparency and gas-barrier properties, exhibiting excellent resistance to mechanical shock and being easily recyclable, comprise a polyester base film (for example shaped from polyethylene terephthalate) 5 μm to 50 μm in thickness, coated on at least one of its two face surfaces with a layer of polyvinyl alcohol having a number-average degree of polymerization equal to or greater than 350 and a thickness less than or equal to 0.3 μm, the mean roughness Rz of the base film being less than or equal to 0.30 μm on the at least one face surface of the film onto which a polyvinyl alcohol layer is coated and said at least one coated face surface comprising, on average, not more than 20 peaks of a height equal to or greater than 1 micrometer and not more than 150 peaks of a height ranging from 0.4 to 1 micrometer, per square millimeter, and such composite films exhibiting a permeability to oxygen, measured at 23° C. at 50% relative humidity, less than or equal to 3 cm 3  /m 2  /24 h.

This application is a divisional of application Ser. No. 08/414,382 nowU.S. Pat. No. 5,658,676 filed Mar. 31, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to polyester-based composite films havinggood gas-barrier properties, and, more especially, to polyester/PVA suchbarrier films.

2. Description of the Prior Art

Polyester films, especially of polyethylene terephthalene, are todaywidely employed in the packaging industry, because of their manyadvantages such as their mechanical properties, their transparency,their nontoxicity, their odorlessness and their tastelessness.

However, their gas-barrier properties may limit their use inapplications requiring a high protection of the packaged productsagainst the action of external gases, in particular against the actionof atmospheric oxygen or, conversely, may not allow the gas compositioninside the packages to be maintained constant.

In order to overcome this disadvantage, GB-A-1,126,952 describes thedeposition of a solution of polyvinyl alcohol ("PVA") onto a polymerfilm to create a polyvinyl alcohol layer, since this latter polymer hasgood gas-barrier properties. To provide good adhesion of the polyvinylalcohol layer to films of cellulose acetate, polycarbonate orpolyethylene terephthalene, an intermediate layer of an adhesive made ofpolyurethane is deposited between the base film and the polyvinylalcohol layer.

EP-A-0,254,468 also describes a composite film comprising a base film ofa synthetic thermoplastic polymer such as a polyamide, polyethylene,polypropylene or polyester, having two coatings on the same single facesurface to the base film; the first coating is adjacent to the base filmand comprises a urethane primer in a solvent, which in the dry statepermits a dispersion of polyvinyl alcohol in aqueous solution to wet theprimer, and the second coating is deposited over the dried and free facesurface of the first coating and includes a polyvinyl alcohol-basedmaterial employed as a gas barrier.

In the composite films of the prior art the polyvinyl alcohol coating istypically separated from the base film by an adhesive layer comprising apolyurethane. Moreover, the polyvinyl alcohol layer is generally thick(for example ranging from 0.15 to 2 micrometers, per the claims of theaforesaid EP-A-0,254,468), and this complicates the recyclability of thecomposite films.

SUMMARY OF THE INVENTION

Accordingly, a major object of the present invention is the provision ofimproved polyester films having good gas-barrier properties provided bya polyvinyl alcohol coating or layer deposited directly onto at leastone face surface of the polyester film in the absence of anyintermediate adhesive layer.

Briefly, the present invention features a polyester-based, biaxiallydrawn or oriented composite film having improved gas-barrier properties,comprising a polyester base film 5 μm to 50 μm in thickness, coated onat least one of its two face surfaces with a layer of polyvinyl alcoholwhich has a number-average degree of polymerization equal to or greaterthan 350, having a thickness less than or equal to 0.3 μm, the meanroughness Rz of the base film being less than or equal to 0.30 μm on theface surface(s) of the film comprising the polyvinyl alcohol layer andthat or these face surface(s) comprising, on average, not more than 20peaks of a height. equal to or greater than 1 micrometer and not morethan 150 peaks of a height ranging from 0.4 to 1 micrometer, per squaremillimeter, and said composite film exhibiting a permeability to oxygen,measured at 23° C. at 50% relative humidity, less than or equal to 3 cm³/m² /24 h.

DETAILED DESCRIPTION OF BEST MODE AND PREFERRED EMBODIMENTS OF THEINVENTION

More particularly according to the present invention, the peak heightdistribution referred to above to define the surface topography of thepolyester base film may be determined in known manner, by observation inan interferometric microscope permitting the number of interferencerings with a light of known wavelength to be counted. The most typicallyemployed interferometers are the Nomarski or Mirau or Michelsoninterferometers.

The polyester comprising the base film may be selected from amongpolyesters which are typically employed for the preparation of biaxiallyoriented semicrystalline films. These are film-forming linear polyesterscapable of being crystallized by orientation and usually prepared fromone or more aromatic dicarboxylic acids or derivatives thereof (forexample esters of lower aliphatic alcohols or halides) and from one ormore aliphatic glycols. Phthalic, terephthalic, isophthalic,2,5-naphthalenedicarboxylic and 2,6-naphthalenedicarboxylic acids areexemplary of such aromatic acids. These acids may be used in combinationwith a minor amount of one or more aliphatic or cycloaliphaticdicarboxylic acids such as adipic, azelaic or hexahydroterephathalicacids.

Ethylene glycol, 1,3-propanediol and 1,4-butanediol are exemplary ofsuch aliphatic diols. These diols may be used in combination with aminor amount of one or more aliphatic diols of higher carbon content(for example neopentyl glycol) or cycloaliphatic such diols(cyclohexanedimethanol).

The crystallizable film-forming polyesters are preferably alkylenediolpolyterephthalates or polynapthalenedicarboxylates and, in particular,the polyterephthalate of ethylene glycol (PET) or of 1,4-butanediol, orcopolyesters containing at least 80 mol % of ethylene glycolterephthalate recurring structural units. The polyester isadvantageously a polyethylene terephthalate whose intrinsic viscositymeasured at 25° C. in ortho-chlorophenol ranges from 0.6 to 0.75 dl/g.

The polyester comprising the base film must be selected such that itsinitial melting temperature is higher than the temperature to which thebiaxially drawn film comprising the polyvinyl alcohol layer is heatedduring its preparation.

The mean roughness Rz of the base film (as defined in DIN Standard 4768)is less than or equal to 0.30 μm and preferably less than 0.25 μm on theface surface of the film onto which the polyvinyl alcohol layer isdeposited.

It is preferable that, on average, this face surface should not comprisemore than 800 peaks of a height less than 0.4 micrometer, per squaremillimeter.

For industrial applications of the processes for the preparation of thefilms of the invention, in which the machine speed to which the film issubjected is generally greater than 100 meters per minute, it is moreparticularly preferred that the face surface of the base film onto whichthe polyvinyl alcohol layer is deposited should on average not comprisemore than 20 peaks of a height equal to or greater than 1 micrometer andnot more than 100 peaks of a height of from 0.4 to 1 micrometer, persquare millimeter. Finally, even more preferably on an industrial scale,the face surface of the base film onto which the polyvinyl alcohol layeris deposited should not on average comprise more than 500 peaks of aheight less than 0.4 micrometer, per square millimeter. It will beappreciated, however, that when operating at lower speeds, especially intests on a pilot plant or laboratory scale, this preferred embodiment isnot necessary.

The other face surface, designated the back face surface, should havesufficient slip properties to permit easy handling of the film,especially its winding over the various guide rolls during drawingoperations, or its reeling onto itself.

This surface roughness may be provided via a variety of techniques. Oneof the most common procedures entails incorporating inert solid fillersinto the polyester before its conversion into film. These fillers aretypically inorganic filler materials such as, for example, silica,titanium dioxide, zirconium dioxide, alumina, silica/alumina mixtures,silicates, calcium carbonate and barium sulfate. These fillers may alsobe polymer particles.

The volume-median diameter of the fillers generally ranges from 1 to 10micrometers and preferably from 1 to 5 micrometers.

The filler content of the film usually ranges from 0.02% to 1% by weightrelative to the weight of the polyester.

One advantageous embodiment of the invention comprises having differentmean roughnesses Rz on the two face surfaces of the base polyester film,for example one equal to or greater than 0.15 micrometer on the backface surface of the film and one less than or equal to 0.30 micrometer,and preferably less than or equal to 0.25 micrometer, on the facesurface of the base film comprising the polyvinyl alcohol coating. Saidface surface of the base film which has the polyvinyl alcohol coatingpreferably comprising, on average, not more than 20 peaks of a heightequal to or greater than 1 micrometer and not more than 100 peaks of aheight ranging from 0.4 to 1 micrometer, per square millimeter and,still more preferably, not more than 500 peaks of a height less than 0.4micrometer, per square millimeter.

The polyester base film may thus include two layers exhibiting differentsurface properties, especially roughnesses.

The production of such asymmetrical films may be carried out bycoextrusion of two polyesters comprising different filler contents and,if appropriate, different fillers. The polyester employed isconveniently the same for the two coextruded layers and the layer ontowhich the polyvinyl alcohol layer is deposited is not filled. Therelative thicknesses of the two polyester layers comprising the basepolyester film may vary widely.

The unfilled (or less highly filled) layer onto which the polyvinylalcohol is deposited generally has a thickness greater than or equal to0.5 μm, preferably greater than or equal to 1.0 μm.

It is also within the scope of the invention to provide a polyester basefilm exhibiting different surface properties on both face surfaces,employing other means known to this art.

It is thus possible, as described in EP-A-0,378,954, to impart a goodslip to the back face surface of the base film by depositing onto saidback face surface a modified polymer obtained by aqueous phase radicalpolymerization of at least one acrylic monomer and of awater-dispersible polyester derived from at least one aromaticdicarboxylic acid and from at least one aliphatic diol and comprising aplurality of sulfonyloxy groups, especially sodium sulfonate.

As indicated above, the polyvinyl alcohol layer has a thickness equal toor greater than 0.3 μm. If desired, this thickness may be less than orequal to 0.20 μm or even less than 0.10 μm, in order to further improvethe recyclability of the film according to the invention. In practice,it is rare to utilize thicknesses less than 0.05 μm.

The present invention also features composite films such as thosedescribed above, which additionally comprise on one of their facesurfaces a printing or a printing primer layer, or else a heat-sealablelayer on the face surface devoid of the polyvinyl alcohol layer.

The printing layer may be deposited by known printing methods such as,for example, photogravure, flexography or silkscreen printing. Inks innonaqueous solution or dispersion will preferably be employed.

The heat-sealable layer is preferably of polyolefin type (especiallypolyethylenes, polypropylenes and ethylene/vinyl acetate polymers).These composite films may be prepared by adhesive backing with the aidof single- or two-component adhesive, or by extrusion coating. In thislatter instance, the face surface devoid of the polyvinyl alcohol layeris precoated in known manner with a bonding primer.

The composite films thus produced may be used for the manufacture ofpackaging such as sachets, small tub closures, overwraps and the like,intended to contain oxidation-sensitive products. They are veryparticularly well suited for packaging under modified atmosphere.

As indicated above, one of the advantages of film of this type is thatit can be easily recycled, by virtue of the reduced thickness of thepolyvinyl alcohol layer.

The present invention also features a process for the preparation of thefilms described above.

More precisely, it features a process for the preparation of compositefilms based on a polyester film having on at least one of its facesurfaces a layer of polyvinyl alcohol of a thickness less than or equalto 0.3 micrometer, wherein the polyester base film is coated on at leastone of its face surfaces which has a mean roughness Rz less than orequal to 0.30 μm and on average comprises not more than 20 peaks of aheight equal to or greater than 1 micrometer and to not more than 150peaks of a height from 0.4 to 1 micrometer, per square millimeter, usingan aqueous solution of polyvinyl alcohol exhibiting at least 95% ofvinyl alcohol recurring structural units, said polyvinyl alcoholexhibiting, in aqueous solution at a concentration of 4% and at 120° C.,a viscosity equal to or greater than 4 mPa.s, and further wherein thecoated film is heat-treated at a temperature equal to or greater than170° C.

It is preferable that the face surface of the polyester film onto whichthe polyvinyl alcohol is coated additionally does not comprise, onaverage, more than 800 peaks of a height less than 0.4 micrometer, persquare millimeter.

Also as indicated above, in an industrial scale application of theprocess according to the present invention at a high machine speed(generally more than 100 meters per minute), it is advantageous that thecoating with the aqueous solution of polyvinyl alcohol be carried out onat least one face surface of a polyester film on average comprising notmore than 20 peaks of a height equal to or greater than 1 micrometer andnot more than 100 peaks of a height ranging from 0.4 to 1 micrometer,per square millimeter.

Lastly, also in respect of an industrial scale application, it is evenmore particularly preferred that the coating with the aqueous solutionof polyvinyl alcohol be carried out on at least one face surface of apolyester film additionally comprising, on average, not more than 500peaks of a height less than 0.4 micrometer, per square millimeter.

The coating of the polyester base film with a solution of polyvinylalcohol may be carried out in-line, or by reprocessing. In-line coatingis preferably employed.

Even more preferably, the coating will be carried out in-line onto amonoaxially drawn polyester base film which again will be drawn, aftercoating, at least in the direction opposite to that of the firstdrawing.

Before the coating of the base film the surface of the latter isgenerally subjected to a physical treatment (such as, for example,corona, flame or plasma) intended to ensure a good spreading of thepolyvinyl alcohol layer over and onto the base film. This treatmentpermits the surface tension of the monoaxially drawn film to be adjustedto a value that is preferably greater than that of the polyvinyl alcoholcoating and preferably to a value equal or greater than 54 mN m.

The aqueous solution of polyvinyl alcohol which is used generally has aconcentration of 1% to 20% by weight per weight and preferably from 5%to 15% by weight per weight. The solution is carefully prepared, firstcold with simple stirring and then by heating to a temperature notexceeding 95° C.; it is filtered after cooling. This solution must befree from gels. the absence of gels may be determined by measuringturbidity, solids content and refracture index.

The polyvinyl alcohol according to the present invention is acommercially available polymer. It may be employed as is, or may beprepared, especially, by hydrolysis of vinyl carboxylates, moreparticularly polyvinyl acetates or copolymers thereof which are rich invinyl acetate recurring structural units, such as vinyl acetate/ethylene(or EVA) copolymers. As indicated above, the polyvinyl alcohol employedcomprises at least 95% of vinyl alcohol units (degree of hydrolysis ofat least 95%). It preferably comprises at least 97% thereof and stillmore preferably at least 98%.

The polyvinyl alcohol comprising the coating of the composite film ofthe invention preferably has a viscosity in aqueous solution at aconcentration of 4% and at 20° C., measured in an apparatus ofBrookfield type, which is equal to or greater than 4 centipoises (or 4mPa.s), which corresponds, within the experimental errors associatedwith the measurements, to a number-average degree of polymerizationequal to or greater than 350.

The selection of the concentration of polyvinyl alcohol in the solutionand of the apparatus used for the coating is determined especially bythe desired thickness of the final polyvinyl alcohol layer.

Without intending to be limited to this particular embodiment, thein-line coating is advantageously carried out with the aid of rollswhich are photoengraved according to the so-called "reverse gravure"technique.

While also not limiting, the polyester base film is in most instancesdrawn or oriented lengthwise (namely, in the machine direction) prior tothe in-line coating using the aqueous polyvinyl alcohol solution.

This drawing (stretching) may be carried out in one or more stages, asalso can the drawing following the in-line coating.

The temperature at which the coated film is treated preferably rangesfrom 180° C. to 240° C. and more preferably from 200° C. to 230° C.

The polyester base may be prepared by extrusion of a polyester asdescribed above, comprising one or a number of inert fillers, inparticular to impart a sufficient slip thereto. It may also be preparedby coextrusion, on the one hand, of an unfilled or more slightly filledpolyester and, on the other, of a filled polyester.

In this second alternative embodiment the polyester base film comprisesa rough filled layer defining the so-called back face and an unfilled,or more slightly filled, layer defining the so-called front face whichwill receive the coating of the polyvinyl alcohol solution.

The filled polyester layer typically has a mean roughness Rz which isgreater than or equal to 0.15 micrometer and preferably greater than orequal to 0.30 micrometer, whereas the unfilled or more slightly filledfront layer has a total roughness less than or equal to 0.30 micrometerand preferably less than or equal to 0.25 micrometer. As indicatedabove, the face surface of the base film onto which the coating of thepolyvinyl alcohol solution will be applied does not comprise, onaverage, more than 20 peaks of a height equal to or greater than 1micrometer and not more than 100 peaks of a height of from 0.4 to 1micrometer, per square millimeter, and even more preferably not morethan 500 peaks of a height lower than 0.4 micrometer, per squaremillimeter.

The composite films of the invention have an excellent resistance tomechanical forces such as, in particular, creasing. This reflects thattheir barrier properties described above are not, or not significantly,decreased or degraded after such a mechanical shock.

In order to further illustrate the present invention and the advantagesthereof, the following specific examples are given, it being understoodthat same are intended only as illustrative and in nowise limitative.

EXAMPLES

General Procedure:

A composite film of polyethylene terephthalate (PET) was prepared bycoextrusion, on the one hand, of a PET containing 0.0800% of silicaobtained by precipitation of gels and exhibiting a mean diameter of 3.3micrometers (measurement using a laser particle size analyzer marketedunder the trademark Sympatec and of Helos type), which formed the basefilm and, on the other, of an unfilled PET whose external face surfacereceived the polyvinyl alcohol (PVA) coating.

The polyester film had a thickness of 12 micrometers in the variousexamples or comparative tests, unless indicated otherwise, and a meanroughness Rz of 0.45 micrometer on the back face surface. It included anunfilled layer (receiving the PVA coating) which had a thickness of 0.5to 2.0 micrometers according to the examples, in order to vary thedistribution of peak heights per square millimeter of the surfaceintended to receive the PVA coating.

The coextruded film was first drawn or oriented lengthwise with a drawratio of 3.4; it was then subjected to a corona treatment which adjustedits surface tension to 58 mN m.

The face surface of the film comprising the unfilled layer was thencoated using a photogravure coating system with an engraved roll. Themachine speed during the coating was 200 m/min. The coating was carriedout using an aqueous PVA solution at a concentration of 10%, carefullyprepared and free from gels. Unless indicated otherwise, the PVAemployed contained 98-99% of vinyl alcohol recurring units and had aviscosity of 5.5 mPa.s (measured at 20° C. on an aqueous solution at aconcentration of 4% using a Brookfield LV viscometer).

The coated film was then subjected to a transverse drawing with a ratioof 4 and was then heat-treated at a temperature of 230° C. (unlessindicated otherwise). The thickness of the PVA layer of the biaxiallydrawn film was 0.10 micrometer (unless indicated otherwise).

EXAMPLES 1 to 3 and Comparative Tests a, b and c

These various tests were carried out using composite films, the facesurfaces of which that received the PVA coatings exhibited differentpeak height distributions. Test a was carried out without PVA coating.Permeability to oxygen was measured at 23° C. at 50% relative humidityfor each of the films; it is expressed in cm³ /m² /24 h (measurementscarried out using an apparatus of Oxtran trademark, type 300H, marketedby Modern Control Inc.).

Table 1 below reports the peak height distributions per squaremillimeter and the mean roughness of the film face surface receiving thePVA coating, and the values of permeability to oxygen.

                  TABLE 1                                                         ______________________________________                                                 Number of peaks of height                                                                   between        Permeability                            Examples                                                                             Rz      >1 μm                                                                              0.4 and 1 μm                                                                        <0.4 μm                                                                          to oxygen                               ______________________________________                                        Example 1                                                                            0.2     4       12       170   1                                       Example 2                                                                            0.3     13      65       160   2                                       Example 3                                                                            0.3     13      60       324   2                                       Test a 0.2     4       12       170   110                                     Test b 0.45    29      110      1,000 5                                       Test c 0.5     32      325      1,630 10                                      ______________________________________                                    

EXAMPLE 4 and Comparative Test d

The procedure of Example 1 was repeated, but the heat-treatment of thePET film was carried out at a different temperature.

Table 2 below reports the measured permeabilities to oxygen, accordingto the treatment temperature adopted (Example 1 was repeated forpurposes of comparison).

                  TABLE 2                                                         ______________________________________                                                      Treatment Permeability to                                       Examples      temperature                                                                             oxygen                                                ______________________________________                                        Example 4     180° C.                                                                          1.3                                                   Example 1     230° C.                                                                          1                                                     Test d        150° C.                                                                          5                                                     ______________________________________                                    

Comparative Tests e and f

These tests were carried out using the same film as in Example 1, butthe coating was a polyvinyl alcohol which had a percentage of vinylalcohol recurring units (or degree of hydrolysis) less than 95%. Thepermeability obtained with the films thus prepared is reported in Table3 below (Example 1 was repeated for purposes of comparison).

                  TABLE 3                                                         ______________________________________                                                    Degree of hydrolysis                                                                        Permeability to                                     Examples    of PVA        oxygen                                              ______________________________________                                        Test e      86.5-89.5     32                                                  Test f      78-81         85                                                  Example 1   98-99         1                                                   ______________________________________                                    

EXAMPLE 5 and Comparative Test g

These tests were carried out using the same film as in Example 1, butthe coating was a polyvinyl alcohol which had a viscosity (measured at20° C. in an aqueous solution at a concentration of 4% with the aid of aBrookfield LV viscometer) other than 5.5 mPa.s (the correspondingnumber-average degree of polymerization DPn, provided by the producer ofthe PVA, is also recorded). The permeability obtained with the filmsthus prepared is reported in Table 4 below (Example 1 was repeated forpurposes of comparison).

                  TABLE 4                                                         ______________________________________                                                  Viscosity           Permeability to                                 Examples  in mPa · s                                                                        DPn    oxygen                                          ______________________________________                                        Example 5 4.5          400    2                                               Test g    3.5          300    8                                               Example 1 5.5          500    1                                               ______________________________________                                    

EXAMPLE 6

The procedure of Example 1 was repeated, the film having a totalthickness of 40 μm and an unfilled layer receiving the polyvinyl alcoholcoating having a thickness of 2 μm.

The permeability to oxygen of the film coated with an 0.1 μm layer was1.5 cm³ /m² /24 h.

While the invention has been described in terms of various preferredembodiments, the skilled artisan will appreciate that variousmodifications, substitutions, omissions, and changes may be made withoutdeparting from the spirit thereof. Accordingly, it is intended that thescope of the present invention be limited solely by the scope of thefollowing claims, including equivalents thereof.

What is claimed is:
 1. A process for the preparation of apolyester-based, biaxially oriented composite film having good gasbarrier properties, comprising: coating a polyester base film on atleast one of its two face surfaces with an aqueous solution of polyvinylalcohol which comprises at least 95% of vinyl alcohol recurringstructural units, said polyvinyl alcohol exhibiting, in aqueous solutionat a concentration of 4% and at 120° C., a viscosity of at least 4mPa.s; and, heat-treating the coated film at a temperature of at least170° C.
 2. The process as defined by claim 1, comprising coating amonoaxially oriented polyester base film with said aqueous PVA solution,and thereafter biaxially orienting the coated film.
 3. The process asdefined by claim 1, said aqueous solution of polyvinyl alcohol having aconcentration of 5% to 15% by weight, and said polyvinyl alcoholcomprising at least 98% of vinyl alcohol recurring structural units. 4.The process as defined by claim 1, comprising heat-treating the coatedfilm at a temperature ranging from 200° to 230° C.